The present invention generally relates to integrated opto-electronic devices and waveguide circuits and more particularly to the hybrid integration of the active and passive optical elements and electronic chips on a common substrate.
Large-scale integration of optical devices and elements is desirable to decrease the cost and increase the functionality of optical and opto-electronic devices and circuits. Two major integration methods are currently under the developmentxe2x80x94the monolithic approach and the hybrid approach. In the monolithic approach, all components such as laser sources, passive waveguide devices, detectors and so on are made on a common substrate from the same material such as silicon or InGaAsP series. In the hybrid approach, different components made of possibly different materials are mounted on a common substrate and are connected to each other by optical waveguides fabricated on the same substrate. In the hybrid approach, the waveguides are made of high-silica glasses that show very low propagation loss, whereas in the monolithic approach the waveguides are made of the same materials as the other components which are normally semiconductors and therefore are highly absorptive. Moreover monolithic integration of optical components may become very expensive since the fabrication process of the different optical elements are not compatible and could be destructive to one another resulting in a low yield.
Therefore, the hybrid integration is a more feasible yet more versatile approach. In U.S. Pat. No. 4,735,677, issued in April 1988, a method for fabricating hybrid optical integrated circuits is disclosed. In that method, optical fibers and semiconductor devices (e.g., laser diodes and detectors) are disposed on predetermined positions on a board and are aligned to pre-made waveguides by pre-made guides. The waveguides and guides are made on a substrate in one process and then the optical elements are placed on the pre-determined positions between the aligning guides. Positioning the devices into the guides and the optical alignment of the devices with the pre-made optical waveguide become very critical and time consuming resulting in expensive hybrid integrated circuits.
In U.S. Pat. No. 5,854,868 issued on Dec. 29, 1998, there is a disclosure of another fabrication method for hybrid integration of optical devices to waveguides. In that method, optical devices are first mounted on predetermined positions, while optical waveguides with a desired refractive index profile are fabricated afterwards. Provided that the optical devices are mounted on their ideal positions, one can quickly align the waveguides to their respective optical devices by means of markers and using a mask aligner. However, the optical devices are mounted manually on their predetermined positions by means of a solder, where it is very difficult to control the position of the mounted devices within the necessary precision, e.g. less than a micrometer. Therefore, this method will also suffer from misalignment errors.
In U.S. Pat. No. 5,737,458 and U.S. Pat. No. 5,562,838 adaptive lithography is employed to interconnect the optical elements already mounted on the substrate. In these disclosures, a laser milling process is proposed to create micro-channels for forming waveguides by filling the channels with a material with a slightly higher refractive index or to etch away the surface of the substrate coated with the waveguide material to form the channel waveguides for interconnection. However, laser milling or ablation may damage the optical elements mounted on the substrate and also results in waveguides with rough surfaces, which will have high scattering losses. Moreover, the laser ablation process employed in that method does not necessarily provide sufficient precision required for the optical alignments.
There is, therefore, a need for a method of hybrid integration that can solve the problem of incompatibility of the fabrication method and the alignment of the optical devices in a cost effective and feasible manner.
It is therefore an object of the present invention to provide a feasible method of hybrid integration of optical and electronic devices and components suitable for reducing misalignment errors.
To circumvent the problems mentioned above, the present invention provides a method for seamless hybrid integration of optical and/or electronic devices on a substrate. The method comprises of forming at least one layer of high silica glasses on a substrate and with mounted optical elements thereon. Optical elements in this invention include, but are not restricted to, devices such as lasers, detectors, lenses, prisms, isolators, waveguide bends, integrated optic devices with input and output, and the like. A device in this invention refers to any one of optical elements, electronic components and integrated circuits, as well as other opto-electronic devices.
At least one of the layers is a photosensitive glass layer in which refractive index or solubility is changed by exposure to a particular spectrum of electromagnetic radiation such as Ultra Violet (UV) light. The photosensitive layer is used as the core region of the waveguide interconnection circuit that will be printed by UV exposure. The substrate is etched or carved at the predetermined position with the predetermined depth such that the emitting and receiving areas of the optical devices are aligned vertically to the core layer level (i.e. the photosensitive layer).
The photosensitive layer may or may not be covered by another layer or layers of silica glass, which are almost transparent to that particular spectrum of electromagnetic radiation.
The substrate with deposited layer or layers of thin film glasses and with the optical elements mounted in the pre-made cavities is masked with a masking material such as chromium. Waveguides or other optical element patterns are then written on the upper layer by a suitable way such as adaptive electron-beam lithography. The substrate with the mask attached on it is then exposed to the light radiation (e.g. UV light) so that waveguides are created in the core layer and optical elements become optically interconnected. The mask pattern may contain lenses and micro prisms that will also be fabricated at the same time at the desired places or junctions. The spot-size of the different optical devices is matched either by the integrated lens or by appropriate tapering of the waveguide widths in the lateral axis and is vertically controlled by double exposure or by further selective exposure to a UV radiation. Alternatively, the waveguides are written by direct laser writing by means of moving either the laser beam or moving the substrate under the laser beam to fabricate the necessary interconnections.
For a precise alignment of optical waveguides to the optical devices, the exact position of mounted devices is detected by an imaging system that can then control a laser beam or an electron beam over the substrate. For instance, the image processor recognizes the actual positions of the devices on the substrate and transfers the pattern to a computer-controlled electron beam lithography machine to cerate a mask on the upper layer that will perfectly align the waveguides to the optical devices.
In accordance with a first aspect of the invention, there is provided a method for integrating at least one device on a substrate, comprising the steps of:
(a) coating a first side of said substrate with at least one layer including a core layer suitable for guiding light;
(b) carving at least one recess in said first side of the substrate and said at least one layer;
(c) mounting the at least one device in said at least one recess; and
(d) forming an optical waveguide coupled to said at least one device within said core layer.;
wherein said optical waveguide corresponds to a position of said at least one device relative to the substrate.
In accordance with a further aspect of the present invention there is provided a method for integrating at least one device on a substrate, comprising the steps of:
(a) coating a first side of said substrate with at least one layer including a core layer made of a photosensitive material;
(b) carving at least one recess in the first side of said substrate and said core layer;
(c) depositing an electrical connector in said at least one recess;
(d) mounting at least one device in said at least one recess;
(e) obtaining an image of the first side of said substrate with said at least one device;
(f) forming a mask in accordance with said image using electron beam lithography; and
(g) exposing a portion of the core layer to a light beam through said mask to shape an optical waveguide through a change in refractive index of exposed portion of the core layer.
The at least one layer preferably includes an inner cladding layer between the substrate and one side of the core layer, and an outer cladding layer at another side of the core layer.
Preferably, an electrical connector is deposited in the at least one recess after the carving step and before the forming step. Following the mounting step, a portion of the recess surrounding the at least one device is preferably filled by a photosensitive material, followed by forming at least one optical deflector within the photosensitive material positioned between the at least one device and the optical waveguide.
In an embodiment of the present invention, an optical waveguide is formed by the steps of obtaining an image of the first side of the substrate to identify a position of said at least one device relative to the substrate, and then defining a shape of the optical waveguide in accordance with the position of said at least one device. The definition of waveguide shape is conveniently determined with the aid of at least one positioning marker by forming a mask in accordance with the position of said at least one device, and then exposing a portion of the core layer to an ultraviolet light beam through said mask to effect a change in refractive index of an exposed portion of the core layer and to form the optical waveguide through said exposed portion. The mask can practically be formed with either electron beam lithography, or by direct laser writing of the optical waveguide on the substrate in accordance with the position of said at least one device.
In accordance with another aspect of the present invention, there is provided a hybrid integrated circuit comprising:
(a) a substrate having a first side coated with at least one layer including a core layer suitable for guiding light;
(b) at least one recess carved in said first side of said substrate and said at least one layer;
(c) at least one device mounted in at least one recess;
(d) an optical waveguide coupled to said at least one device;
wherein said optical waveguide is formed within said core layer to correspond a position of said at least one device relative to the substrate.
Preferably, a portion of the recess surrounding the at least one device is filled by a photosensitive material.
The hybrid integrated circuit can also have at least one optical deflector formed within the photosensitive material and positioned between the at least one device and the optical waveguide. The at least one optical deflector can either be a lens or a micro-prism. In addition, the integrated optical circuit can include at least one optical fiber terminal coupled to the said at least one optical waveguide, and adapted to receive an optical fiber having a core aligned to the core layer. Practically, this optical fiber is embedded within the core layer.
The at least one device can include a laser diode, and/or a modulator. The optical waveguide can also have a Bragg grating section. Additional elements that can be integrated within the hybrid circuit include an electronic integrated circuit mounted in the recess and electrically coupled to the at least one device.
In summary, the present invention provides a method of fabricating and interconnecting optical devices with state-of-the-art alignment precision using a common board and applying adaptive electron beam lithography or direct laser writing. Furthermore, the invention reveals a method for formation of lenses, micro-prisms, spot-size adapters, and gratings incorporated in waveguide interconnections of mounted optical devices. The method is suitable for low-cost volume production of hybrid integrated optical circuits and devices.